Semiconductors partially substituted with magnetic ions are called diluted magnetic semiconductors (DMSs) and are expected to be useful in spintronics devices, where electron spins can be controlled by electric field and/or by photons. Ferromagnetic DMS's with Curie temperatures (\(T_C\) 's) higher than room temperature are highly desirable for the development of spintronic devices. To date, much work in this area has been done, mainly on II-VI and III-V compounds doped with magnetic ions such as (Cd,Mn)Te [1]} and (Ga,Mn)As [2]}, [3]}, but their \(T_C\) 's are far below room temperature. Ferromagnetism was observed in Mn-based zinc-blende II-VI compounds such as (Cd,Mn)Te after the result of carrier induced ferromagnetism [4]}. Kuroda et al.[5]}, [6]} reported that Cr rich phases of (Zn,Cr)Te showed room temperature ferromagnetism, causing a stimulation of II-VI DMS. Ferromagnetism was also observed in (Ga,Mn)As [2]}, [3]}. It was theoretically suggested [9]} that the co-doping of magnetic semiconductors with shallow impurities affects the self-assembly of magnetic nanocrystals during epitaxy, and therefore modifies both the global and local magnetic behavior of the material. This concept was also qualitatively corroborated by experimental data for (Cd, Mn, Cr)Te [10]} and (Ga, Mg, Fe)N [11]}, [12]}, [13]}. However, origin of ferromagnetism at room-temperature is controversial so far. Recently, Matsumoto et al.[14]}, [15]} reported the occurrence of room temperature ferromagnetism in Co-doped anatase TiO\(_2\)  films. According to Fukumura et al. [16]}, [17]} the high electron carrier densities and Co content favor the ferromagnetic phase in Co-doped rutile TiO\(_2\)  at 300 K. Room-temperature ferromagnetism was also reported in such materials as (Ga,Mn)N [18]} and (Al,Cr)N [19]}. The near edge x-ray absorption fine structure study of Co-doped TiO\(_2\)  by Griffin et al. [20]} claims that ferromagnetism is due to \(d\) -\(d\)  double exchange mediated by tunneling of \(d\)  electrons within the impurity band. Some studies that also claim the ferromagnetism of Co-doped TiO\(_2\)  is due to Co metal clusters [21]}, [22]}, [23]}, [24]}. The recent theoretical study by Calderon et al. [25]}, electric field-induced anomalous Hall effect (AHE) study by Yamada et al. [26]} and x-ray photoemission spectroscopy study by Ohtsuki et al. [27]} suggested the ferromagnetism of Co-doped TiO\(_2\)  is due to carrier mediated. However, direct information about the magnetization as a function of carrier density has been lacking. Soft x-ray magnetic circular dichroism (XMCD) at the Co 2\(p\rightarrow 3d\)  absorption (Co \(L_{2,3}\) ) edge is a powerful technique to clarify this issue because it is an element-specific magnetic probe [24]}. A previous XMCD study on rutile Co-doped TiO\(_2\)  by Mamiya et al. has revealed that the ferromagnetism is not due to segregated Co metal clusters but is due to Co\(^{2+}\)  ions in the TiO\(_2\)  matrix [29]}. However, the XMCD signal intensities were an order of magnitude lower than that expected from the bulk magnetization [29]}. In a more recent work [17]}, we performed x-ray absorption spectroscopy (XAS) and XMCD studies on rutile Co-doped TiO\(_2\)  not only by the surface-sensitive total electron yield (TEY) mode but also the bulk-sensitive total fluorescence yield (TFY) mode and found that Co ions in the bulk indeed have a large magnetic moment of 0.8-2.2 \(\mu _B\) /Co.
